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German study gives green light to microwaving metal

A German scientific institute study which saw a range of foods packaged in five different-shaped steel and aluminium containers test cooked in four microwave ovens of differing nominal powers has concluded microwave cooking metal packs is safe and gives good results providing certain “procedures” are followed.

With food manufacturers increasingly seeking to differentiate food products like chilled ready meals by presenting them in quick, easy-to prepare packaging formats, the Fraunhofer Institute for Process Engineering and Packaging researched how well and safely foodstuffs including tap water, egg batter, chilli con carne and an infant pasta with vegetables and meatballs, all packed in metal containers, cooked in 700 800, 900 and 1000w microwaves. The tested containers were: a round steel bowl (99mm diameter x 35mm height), with a 200g filling; a round steel bowl (127mm diameter x 30mm height) with a 250g filling; a square steel container (125mm x 125mm x 25nmm) with a 300g filling; a rectangular aluminium container (160mm x 99mm x 35mm), with a 400g filling, and a round steel container (153mm diameter, 36mm height) used with a 425g filling.

All the packs were selected for their large open surface and shallow profile, characteristics considered preferable for optimal microwave oven use. Comparison experiments were performed with similarly-sized and shaped C-PET containers, filled with the same quantity of test food. Oven power setting was 100% throughout, with only cooking times adjusted to achieve the heating goal.

In some 1,000 microwave heating experiments with the metal containers (this quota included tests undertaken during a previous Fraunhofer study), “not one spark was reported, nor a risky situation observed”. However, the researchers found that, for safe metal container use, a number of “procedures” need to be applied. These included: removing metal lids before heating; only placing full containers in the oven; heating only one container at a time, and careful container positioning in the turntable’s centre – with an “insulating air gap” of at least 2.5cms between pack and oven walls

.While the tests proved metal packs can be safely microwaved, they also underlined earlier research’s findings that heating efficiency is lower than with similar sized-plastic packs, meaning longer heating times. This is largely because, with metal containers, microwave energy only reaches the food from the “open surface” side while, with plastic packs, microwaves are absorbed from all sides. With some metal containers, twice the heating time was needed for the same volume of food in a plastic pack. Large metal containers also heated more efficiently than smaller ones.

The researchers also evaluated suggestions microwaving metal packs can reduce the life of the ovens’ magnetron element.

However some 250 experiments with different-sized metal containers, including “misuse experiments” with empty ones, produced no evidence.

One interesting finding was that temperature distribution during cooking was “generally more uniform” in the metal packs than in the plastic container tested.

The study, “Microwaveability of Steel and Aluminium Food Packaging”, was funded by organisations including Alcan Rhenalu, the Association of European Producers of Steel for Packaging (APEAL), Crown Food Europe, Fördergesellschaft Metallverpackungen Düsseldorf, Impress and Novelis Deutschland.

The Fraunhofer study effectively gave the green light in safety and heating efficiency terms to microwave cooking of metal packs